In recent years there are CD-ROM drives for use in computer peripheral equipment which are capable of high-speed reproduction, such as quadruple-speed reproduction, octuple-speed reproduction and further decuple-speed reproduction. In the case of an apparatus capable of high-speed reproduction of discs on which signals are recorded in a CLV (Constant Linear Velocity) system, a rotation speed of a disc motor varies according to a radial position of a currently read track of the disc. The rotation speed decreases inversely to a radial position of a track. Accordingly, when a track jump operation is to occur, if a normal rotation speed for a target track significantly differs from the rotation speed of the current reading operation, a motor speed must be greatly changed. As a result, a relatively long time is needed to reach the normal rotation speed of the target track.
To reduce an average access time for a track jump operation, an approach of using a disc motor having an enhanced acceleration/deceleration performance was proposed. Thus, the disc motor is capable of producing a high torque. However, such a disc motor having the enhanced acceleration/deceleration performance largely impedes the realization of using small-sized, cheaper CD-ROM drives. Further, if such a CD-ROM drive adapted for such high-speed reproduction is mounted in small portable information handling equipment, users may feel unpleasant because of the mechanical vibrations in the CD-ROM drive due to the high speed rotation of the disc motor.
As another approach for achieving high-speed reproduction without affecting the performance of disc motors, a CAV (Constant Angular Velocity) system reproduction is studied for discs on which signals are recorded by the CLV system. In this approach, because the rotation speed of a disc motor is constant at all track positions on a disc, it is unnecessary to employ a disc motor which has an enhanced acceleration/deceleration performance.
In this case, however, a data reproduction rate varies over a wide range from an innermost track area and an outermost track area of the disc. The data reproduction rate increases as the reproduction progresses toward the outer track. Accordingly, if a motor is rotated at a high speed in order to achieve the high-speed reproduction, a data processing circuit is required to have a much higher data processing capacity. However, because there is an upper limitation to the oscillation frequency of a clock generator which defines the performance of a data processing circuit, there is also a limitation for improving a reproduction speed of a disc reproducing apparatus.
In a conventional disc reproducing apparatus, such as CD-ROMs, the need for high performance disc motors was a stumbling block toward miniaturization, weight-reduction and cost-reduction of the apparatus. In addition, the mechanical vibration occurring in the apparatus when a disc motor is driven at high speed for high-speed reproduction was also a problem which could not be neglected.